1.1 Technical Field
The present invention relates to a novel coating for metal surfaces and a method for implementing the coating. The invention particularly relates to a three-layer coating for metal surfaces, comprising a primary epoxy layer, an adhesive layer and a polyolefin layer. The invention also relates to a method for coating metal surfaces.
1.2 Description of The Related Art
Three-layer coatings for metal surfaces are already known in the related art, notably for coating metal tubes. The first layer consists of an epoxy primer which initially forms a gel and then cross-links or sets. The second layer consists of a polymer adhesive, and is generally arranged over the primer before the latter gels. The third layer generally consists of a thermoplastic polymer, most frequently a polyolefin.
Thus, the following patents disclose such three-layer systems, which enable the advantages of epoxy resins and polyolefins to be combined, specifically high adherence, good shock resitance and cathodic disbondment resistance: EP-A-0057823, EP-A-0205395, FR-A-2184321, and FR-A-2529829. However, in these patents, there is no mention of problems associated with epoxy/adhesive reactivity and to the time that passes between application of the epoxy layer and the adhesive layer.
WO-92/03234 discloses a method for coating metal tubes in which the adhesive is applied over a partially cross-linked epoxy resin so as to favor epoxy/adhesive reactions and thus adhesion. It is additionally recommended to apply the adhesive before cross-linking, but after the epoxy gels. However, no example or numerical value is given.
Additionally, when the adhesive is arranged over the epoxy primer after it gels, salt water endurance tests show that, in the case of these conventional three-layer systems, total disbondment occurs at the metal/epoxy interface, accompanied by significant corrosion. It is essential to avoid this negative effect in many applications, notably the transport of hydrocarbons through underwater pipes. If, on the other hand, the adhesive is arranged over the primer after the latter gels, then salt water endurance tests show that no more disbondment occurs at the metal/epoxy interface, nor does the associated corrosion; however, this time, disbondment at the epoxy/adhesive interface is observed.
There is thus a need for coatings that simultaneously have good adhesion and good water endurance properties.
It appears from the related art that the choice of the epoxy/adhesive pair is an essential parameter. Indeed, to obtain high adhesion between the epoxy primer and the adhesive, the latter must be applied rapidly before the primer gels in order to ensure a high level of reaction with the epoxy primer. This means that the time between application of the epoxy and adhesive layers must be relatively short, and below the gelling time of the primer. This criterion is sometimes difficult to satisfy on industrial production lines as a result of their design (proportioning of application equipment, linear speed of the tube) or for large-diameter coating (low rotational speed and linear speed of the tube). There is thus an unmet need for adhesive compositions having high adhesion which is independent of the nature and origin of the primer and/or of the thermoplastic polymer, and also high adhesion which is independent of the time of application between the epoxy-adhesive layers.